10 May 2013
Microplastics and the Threat to Our Seafood
Tracing Pollutants through a Pellet
Old plastic never dies, it just fades away…into tiny pieces called "microplastics." Microplastics are fragments of plastic that measure less than 5 mm (as defined by NOAA). The abundance of microplastics in the oceans has grown steadily over the last few decades, as plastic use continues to rise. While a major portion of microplastics comes from the degradation of plastic products into smaller fragments, I have focused on the small resin pellet that is the industrial feedstock of plastic products. Since the pellets are durable and accumulate persistent organic pollutants (POPs) in the environment, they are a good vehicle to track these pollutants and how they enter the food web.
Cylindrical or disk-shaped pellets are shipped to factories all over the world, and poured into molds to make plastic bottles, caps, bags and packaging. Unintentionally, the pellets are spilled into the environment. These tiny travelers have grown ubiquitous on beaches around the world, even popping up on remote islands such as Cocos Island, Canary Island, St. Helens and Henderson Island. As they float in the sea, pellets accumulate persistent organic pollutants (POPs).
POPs are hazardous human-made chemicals that are resistant to degradation in the environment. Polychlorinated biphenyls (PCBs), different sorts of organochlorine pesticides (e.g. DDTs and HCHs) and brominated flame-retardants are all POPs. Because they are basically lipophilic (i.e. have a high affinity for oils and fats), POPs accumulate in fatty tissues of marine organisms. They have the potential to cause many adverse effects in wildlife and humans (e.g. cancer, malformation, decrease in the immune response, impaired reproductive ability).
Plastic pellets are also lipophilic and have an extremely high affinity for POPs. The concentration of POPs in plastic resin pellets is a million times higher than in the surrounding seawater. We first observed this accumulation through on-site experimentation in 1998 (1). This is why tracking the resin pellet is so valuable.
International Pellet Watch
In 2005, I founded International Pellet Watch (IPW) to track and study plastic resin pellets. We asked citizens across the globe to collect plastic resin pellets from the beaches they visited and send them to our laboratory via airmail. We analyzed the POP content of the pellets, and its global distribution. The results are sent to the participants via email and released on the web.
So far pellet samples from approximately 200 locations in about 40 countries have been analyzed. We always analyze five samples from each location to see piece-to-piece variability. Thus, about 1000 pellet samples have been analyzed so far. POPs were detected in every one of those 1000 pellet samples from around the world. Even in the pellets from remote islands, POPs were detected and some samples occasionally exhibited high concentrations of POPs. This provides evidence that plastic pellets transport POPs for a long distance to remote areas (2).
The advantage of International Pellet Watch is the extremely low cost of sampling and shipping compared with the conventional monitoring methods used for water, sediment and biological samples. Because the pellets concentrate POPs by a million-fold, less material is needed for chemical analysis. Shipping cost is reduced. No special instrument is needed, so sampling costs are also minimal. In this way, IPW has been able to create a global POP pollution map at a very low cost.
In addition, by engaging non-specialists in the process of sample collection and analysis, the IPW has increased public awareness of plastic pollution and the chemical risk associated with POPs in microplastics.
Using pellets we have been able to observe spatial patterns of POP concentrations. For example, PCB concentrations were two to three orders of magnitude higher in highly-industrialized areas (e.g., Los Angeles, Boston, Tokyo, Athens), where a legacy of PCB pollution of PCBs has been observed (3). (See Figure 1) Although usage of PCBs was banned in these countries in the 1970s, they accumulated in the bottom sediments in coastal zones, due to their persistent and hydrophobic nature. They are easily re-suspended and remobilized by physical processes (wind, waves, currents), mixing or stirring of sediments by animals bioturbation and activities such as dredging and underwater construction. In this way, the PCBs in the pellets continue to contaminate coastal waters.
These spatial patterns are consistent with those found by traditional monitoring methods (e.g., mussel watch), indicating the reliability of IPW as a monitoring tool. The spatial pattern of POPs in pellets was also concordant with those in plastic fragments (4). This means that similar accumulation of POPs occurs on plastic fragments as on pellets, confirming that pellets can be considered surrogates for plastic fragments and microplastics in general.
Fragments are even more dangerous
In addition to the absorption of POPs, marine plastics contain additives such as plasticizers, antioxidants, anti-static agents and flame retardants. Some additives and additive-derived chemicals (e.g., nonylphenol, bisphenol A) cause endocrine disruption--that is they interfere with body processes mediated by hormones. The potential damage from this can be impaired brain development, disabilities in learning and behavior, malformations of the body and limbs, disruption of normal sexual development (including feminization of males or masculinization of females) and increased incidents of cancer (e.g. breast and prostate cancers).
While the additives in pellets may be very damaging, even more additives are applied to finished plastic products, making plastic fragments even more hazardous than pellets. Our latest study demonstrated that endocrine disrupter nonylphenols are present even in water bottle caps (5). We surveyed 93 caps from 63 brands sold in 18 countries and detected nonylphenols in 44 samples from several countries including the USA, EU countries, and China. We also detected nonylphenols in plastic caps stranded on beaches (5). Nonylphenols in mineral water bottle caps are just the tip of an iceberg, as various hazardous chemicals derived from additives have been detected in various plastic fragments on the beaches (4).
How do these pollutants enter the food
What happens when the hazardous chemicals in plastics enter marine ecosystems and come in contact or are ingested by marine organisms and ecosystems? More than 180 species of animals are known to have ingested plastic debris, including birds, fish, turtles and marine mammals. Physical damage to the organism by the ingested plastic has been known and reported for many species of organisms (6). What has been less understood is that even iIf the organisms survive the ingestion of microplastics, the particles themselves introduce hazardous chemicals into the bodies of marine creatures. Several studies examined the link between plastic in seabirds’ stomachs and the concentrations of POPs (i.e., PCBs) in their tissues. The evidence of plastic-mediated transfer of PCBs to biological tissues of seabirds was significant but weak (7) (8) (9), only because marine organisms already absorb PCBs though natural food webs.
More recently, we obtained clearer evidence of the transfer of chemicals from ingested plastics to seabird tissues by focusing on two specific POPs, the highly-brominated flame retardants BDE209 and BDE183 (10). We know that plastic-mediated exposure is probably the major route by which higher-trophic-level organisms acquire these two POPs, probably because they bind strongly to particles, their large molecular size limits transport through cell membranes and they are metabolized rather quickly (11) (12). Mizukawa et al. 2009 showed that they do not travel through the food web of bivalves, crabs and fish in Tokyo Bay (13). Therefore, though more observations and better understanding of the detailed mechanisms are needed, we can say that BDE 183 and 209 in marine animals probably comes mainly from plastic fragments and indicates that POPs from plastics are transferred to the internal tissue of animals that ingest them.
So far, evidence of transfer of chemicals from microplastic particles to
biological tissue has been obtained for relatively large microplastics (i.e. 1
mm to 5 mm). However, the presence
of a smaller range of microplastics (less than 1 mm and perhaps better termed “microscopic
plastics”) in seawater and marine sediments has recently been demonstrated (14) (15). Because their very small size is
comparable to zooplankton, microscopic plastic particles are taken up by filter feeders and thus enter the food web.
More recently, invasion and long-term residence of these microscopic plastics in the circulatory system of bivalve molluscs has been demonstrated (16) (17). Scientists are concerned that chemicals associated with microscopic plastic could be transferred to the internal system of lower-tropic-level organisms such as mussels, oysters or copepods, then biomagnified to animals at higher tropic levels. This would mean not only persistence but increases in levels of toxic chemicals.
There have been alarming signals of unseen threats to marine ecosystems of late. In the north Pacific Central gyre, researchers found six times higher abundance of microplastics than zooplankton (18). Our own work at IPW shows that POP pollution is travelling from plastic pellets into the food chain, of which we partake.
Most plastic enters the ocean from the land. More than half is single-use plastics. There are problems with all of the commonly used practices of waste management. Reduction of plastic waste is essential for a sustainable society. Reduction of single-use plastic is a fundamental and effective way to decrease the risks associated with plastics. Thus, at the International Pellet Watch, our motto is: “No single-use plastic!”
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(16) Browne, M.A., Dissanayake, A., Galloway, T.S., Lowe, D.M., and Thompson, R.C., 2008. Ingested microscopic plastic translocates to the circulatory system of the mussel, Mytilus edulis (L.). Environmental science & Technology 42, 5026-5031. 10.1021/es800249a
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